PRO TOCOCCOIDE& 



411 



In the following paragraphs only the more remarkable or better 

 Tmown genera are described. 



In Sdffdium A. Br., made by some writers the type of a distinct 

 family SCIADIACE^E, the peculiar mode of germination of the zoospores 

 gives rise to a remarkably complicated structure. Each individual consists 

 at first of a single elongated cylindrical cell. The green protoplasmic 

 contents of this cell break up ultimately into a number of biciliated 

 zoospores, which are set free by the upper portion of the cell -wall be- 

 coming detached in the form of a cap. 

 The zoospores do not, however, escape, 

 but germinate while still attached to 

 the mother-cell, giving rise to a cluster 

 of smaller cylindrical cells springing 

 from the apex of the mother-cell. This 

 may go on until the colony consists of 

 as many as four generations, giving 

 the appearance of a minute branching 

 shrub. The zoospores of each genera- 

 tion are smaller than those of the pre- 

 ceding one, and it is probable that 

 those of the last generation, which escape 

 altogether from the parent-cell, are 

 conjugating zoogametes. It is pos- 

 sible that a form allied to Sciadium may have been the starting-point of 

 the Siphonocladaceae, with which family it shows a certain affinity, as, for 

 example, with Valonia. 



Chlorothecium Bzi. (Malpighia, 1888, p. 250) occurs in the form of 

 palmeiloid colonies with a thick and firm cell-wall on aquatic plants. 

 From the cells of these colonies are developed zoosporanges, or rather 

 gametange:, without any alteration of their primitive form ; from each 

 gametange there escape from two to four swarm- spores, or occasionally 

 only one, each provided with a single cilium and a conspicuous red 

 pigment-spot. These swarm-spores are zoogametes, conjugating by 

 gradual fusion. After hibernating the contents of the zygosperm break 

 up into two masses, each of which escapes as a non-sexual zoospore, so 

 that the zygosperm is itself a zoos'porange. From these zoospores are 

 again formed the palmeiloid colonies, in which form Chlorothecium 

 may multiply itself non-sexually without producing zoogametes. 



The position of Halosphara (Schmitz, Mittheil. Zool. Stat. Neapel, 

 1878, p. 61) is very doubtful. Each individual is a minute green globe, 

 just visible to the naked eye,, as much as 0-5 mm. in diameter, floating 

 on the surface of the sea, and bearing an external resemblance to Volvox. 



FIG. 336. Sciadium arbuscnla A. Br. 

 (magnified). 



